The invention relates to a level shifting circuit for maintaining the level of signal to be inputted in the servocircuit in the optical disc apparatus within a given range.
The servocircuit for making tracking and focussing control in an optical disc apparatus may have an automatic gain control device in order to prevent the operation stability from being jeopardized and to prevent the tracking and focussing control from causing errors due to level fluctuation of signals taken out of a photosensor. The automatic gain control device which will be called an "AGC device" hereinafter comprises a divider for forming a reciprocal of the light quantity signal level and a multiplier for multiplying the reciprocal with an error signal in the servocircuit.
The AGC device may be appreciable, but there is inevitably a limitation on the input level of the AGC device itself due to the divider used therein. Although the divider may form the reciprocal, the output thereof can not be raised endlessly when the level of input signal is lowered. An amplifier used for the divider will become saturated. Furthermore, since the divider inevitably has an offset in view of the structure thereof the influence of the offset on the output to appear when the level of input signal is lowered cannot be ignored. For the reasons referred to above, normal operation cannot be assured and the range of input signal level necessary for normal operation cannot be so expanded even if the AGC device is provided, in a case where the input signal level to the servocircuit is low.